Packaged IC units as electronic parts packaged on a printed wiring substrate, including a central processing unit (CPU), generate heat during the operation to raise the temperature, and thereby their performance is lowered or ruined at times. Therefore, it is necessary for the packaged IC units to be equipped with a heat dissipating means. Hitherto, the method of providing a heat dissipater on the surface of a packaged IC unit by pressure welding to utilize convection for improving the heat-dissipating efficiency has been proposed, and put to practical use.
However, such a method cannot avoid leaving gaps in the heat dissipater-packaged IC unit interface, so that the packaged IC unit and the heat dissipater have a reduced contact area in their pressure-welded plane. Accordingly, the heat dissipation efficiency cannot be achieved as it was designed. In order to close up those gaps, it has been carried out to lay a heat-reducing sheet, heat-reducing grease or the like between the packaged IC unit and the heat dissipater (as disclosed, e.g., in Japanese Tokkai Sho 56-28264 (U.S. Pat. No. 4,265,775) and Japanese Tokkai Sho 61-157587, wherein the term "Tokkai" means a "unexamined published patent application). In the case of a heat-reducing sheet, the material for heat reduction is molded in advance into a sheet having a thickness adequate for securing strength. Because of its thickness, the sheet increases in thermal resistance to fail in achieving sufficient heat reduction efficiency.
On the other hand, the heat-reducing grease has the advantage of making it possible to close up the gaps between the packaged IC unit and the heat dissipater by applying pressure thereto even when they have rough surfaces. Hitherto, the heat-reducing grease containing zinc oxide powder, boron nitride (having a hexagonal crystal structure: h-BN) powder or the like as a main component has been proposed. However, such a mineral itself has low thermal conductivity. For instance, the thermal conductivity of zinc oxide is about 20 W/mk and that of boron nitride (h-BN) is about 60 W/mk. Therefore, the, foregoing grease is unsatisfactory in heat-reducing properties.
In addition, the grease containing aluminum nitride having high thermal conductivity of about 320 W/mk as a main component has so far been proposed (e.g., in Japanese Tokkai Sho 52-125506). While the aluminum nitride can ensure satisfactory heat-reducing properties in the grease, it has also the disadvantage of hardness. More specifically, when the grease is applied to a packaged IC unit, such as CPU, having a bare surface, cases are often met with that the aluminum nitride in the grease damages the silicon surface of the packaged IC unit because of its high Mohs's hardness of 7-9 to mar the characteristics of the packaged IC unit.